In order to talk about packages, we’re going to divide them in groups:

core packages

Sublime Text requires these packages in order to work.

shipped packages

Sublime Text includes these packages in every installation, although they are not
technically required. Shipped packages enhance Sublime Text out of the box and
very often they have been contributed by users or other third parties.

user packages

These packages are installed by the user to further extend Sublime Text. They
are not part of any Sublime Text installation and are always contributed by
users or other third parties.

installed packages

Any package of any type that Sublime Text can restore if deleted.

Let’s emphasize that this artificial division is for the sake of clarity in this guide.
Sublime Text makes no distinction between these types of packages internally.

Sublime Text keeps a copy of all installed packages so it can recreate them when
needed. This means it will be able to reinstall core packages, shipped packages
and user packages alike. However, only user packages installed as a sublime-package
are added to the registry of installed packages. Packages installed in alternative
ways will be completely lost if you delete them.

To revert Sublime Text to its default configuration (plus installed packages),
delete the data directory and restart the editor. Keep in mind, though, that
Packages\User is not an installed package and you will lose its contents if
you delete it.

In any case, make sure to back things up before taking an extreme measure like
this.

Ultimately, installing a package consists simply in placing the directory
containing Sublime Text resources under Packages. The only thing that
changes from one system to another is how you copy these files.

Installing Packages vs Installed Packages

Note that installing a package doesn’t actually make that package an
installed package. Installed packages are .sublime-package archives
residing in the InstalledPackages folder. To install a package
means to copy it to Packages.

Packages installed as .sublime-package archives get added automatically
to InstalledPackages, so they are very convenient.

Explaining how to use version control systems (VCSs) is outside the scope of
this guide, but there are many user packages available for free on public
repositories like Google Code, GitHub and Bitbucket.

Packages/Default and Packages/User also receive a special treament when
merging files (e. g. .sublime-keymap and .sublime-settings files). Before
the merging can take place, the files have to be arranged in an order. To that end,
Sublime Text sorts them by name, but files contained in Default and User
are special: Default will always go to the front of the list, and User
to the end.